For years, high carbon rods have been prepared for wire drawing through a heat treating or "patenting" process in which the hot rolled rods are heat treated to optimize the pearlitic microstructure (and thus the tensile strength) of the high carbon rods. These rods are utilized in a variety of industries, such as to produce high carbon wire, mechanical spring wire, wire rope, prestressed concrete strand and the like. The high carbon rod must meet application specific mechanical properties, such as a desired tensile strength, ductility, hardenability and the like. The mechanical properties within rods formed through the parenting process were dependent upon the parenting process itself and the chemical composition of the elements making up the rod (i.e., the rod chemistry).
The rod buyer effected the parenting process as an initial step prior to transforming the rod to a desired end product. The tensile strength of the end product was a function of the buyer's parenting process and the rod's chemistry. Hence, in the industry, it became standard practice for the rod buyers to identify and order application specific rods by designating their chemical compositions in accordance with the AISI grading system, with the expectation of receiving rods having a heat treating response within a preferred range. Once the rods were heat treated or patented, they were transformed such as through a wire drawing operation, to produce the desired end product. As the resulting rod tensile strength is a function of the rod chemistry and the heat treating variables, chemistry, particularly carbon, became the key requirement to be specified by the rod buyer. The different manganese ranges of the AISI grades were generally chosen depending on the type of heat treatment process being used. These element levels represent fixed aim levels.
Once the chemistry was designated by the buyer, the rod supplier adjusted the heat chemistry to meet the "fixed" aim levels for elements designated by the buyer. The raw materials are melted in the furnace, which is tapped to obtain a lot or "heat" of steel. The "heat" of steel is poured into a ladle where it is tested to determine its chemistry (i.e., the percentage content of each element designated by the buyer and any other elements of interest). Next, the sampled element percentages are compared to the buyer designated percentages (fixed aim levels) to determine whether the heat of steel meets the buyer's specification. If not, the rod supplier adds an amount of each element to the ladle necessary to meet the fixed aim levels. In accordance with this process, it may be necessary to vary the quantity of multiple elements. Once the fixed aim levels are achieved, the heat of steel is rolled into rods. Hence, this process produced rods independent of, and without concern for, the mechanical properties of the rod.
In recent years, a new controlled cooling process commonly referred to as the "Stelmor" process has been implemented for producing high carbon rods without the use of a patenting step. Controlled cooling processes utilize a medium, such as air, water, molten salt and the like to supercool hot rolled rods in order to achieve a ferrite/pearlite microstructure having desired mechanical properties. With the introduction of the controlled cooling process, high carbon wire may be produced directly from hot rolled rods. Thus, it is possible to eliminate the patenting process so long as the rod is rolled to a diameter which is not unduly larger than the desired wire diameter. A substantial cost savings results from eliminating the patenting step. However, eliminating the patenting step created the need, within the rod mill, to produce hot rolled rods satisfying critical mechanical properties. Today, hot rolled rods have become useful in industries which are extremely demanding upon the mechanical properties of the rod.
Prior to the Stelmor process, the rod's mechanical properties were dependent upon the rod chemistry and the patenting process, with little consideration being afforded to the rod manufacturing process. However, present day rod mills utilizing a controlled cooling process, typically include a forced air cooling system with the ability to effect substantially the mechanical properties of the hot rolled rod. Thus, by varying the operating parameters of the rod manufacturing process, the rod supplier is able to vary the rod's mechanical properties.
With the advent of the forced air cooling system and the elimination of the heat treating step, the starting rod tensile strength has become a function of the rod chemistry and the rod manufacturing process, both of which are controlled by the rod supplier. Yet, the ordering system has not changed significantly. By necessity, the buyer (wire producer) had to use a trial and error procedure to determine the grades of steel needed for a specific end product (wire drawing practice) to obtain the tensile strengths required. The buyer learned to restrict various chemical element ranges within a grade to obtain better control of the rod tensile strength. The end result is that the buyer became the steel alloy designer. The August of 1993 version of the steel products manual, "Carbon Steel Wire and Rods", a publication of the Iron and Steel Society (which is incorporated by reference) includes a table showing typical average tensile strengths for a rod produced in a controlled cooling system as a function of carbon and manganese levels. From this table, the buyer could presumably estimate carbon and manganese aims to achieve a desired tensile strength.
The problem with this system is that there are additional variables in the rod manufacturing process, such as rolling temperatures, cooling rates, metallic and non-metallic residuals, and grain refining elements that also affect rod tensile strength. These variables may not be covered in typical rod specifications. As a result, the variation in tensile strengths of rods ordered to restricted chemistry ranges is still too large to meet desired tensile ranges consistently. This is particularly true when the buyer orders rod from different suppliers. Different suppliers may have quite different melting, casting, and rolling processes resulting in different rod tensile strengths for the same chemistry specification. Thus, the rod buyer must consider more than just the rod chemistry when specifying the grade of the desired rod with the expectation of the rod having desired mechanical properties.
For example, in the high carbon wire industry, an important mechanical property of a drawn wire is its breaking load or tensile strength. The finished wire tensile strength is dependent upon the wire drawing parameters (e.g., number of passes, amount of reduction per pass, total reduction) which dictate the degree to which the tensile strength of the resulting wire is varied from that of the starting rod. If the tensile strength of the starting rod is too low or too high, the wire drawing parameters cannot be adjusted sufficiently to reach the desired wire tensile strength. Thus, the wire producer must have the correct starting rod tensile strength to meet consistently and predictably the required finished wire tensile strength.
However, designating rod chemistry based upon the AISI specifications did not ensure that the starting rod tensile strength would be within a desired range since the buyer had little control over the rod mill process and particularly the forced air cooling process therein. This uncertainty was further frustrated by the fact that different rod mills used different setups. As these parameters are varied, so is the resulting tensile strength. Thus, the buyer was afforded little security in obtaining a desired tensile strength by designating the general chemistry for such a rod.
The rod supplier has the option to adjust the rolling and cooling parameters of the rod manufacturing process to produce rods having the preferred tensile strength. However, the supplier's ability to effect tensile strength is limited. Further, as the supplier varies the rod manufacturing process parameters, it operates in a non-optimal configuration. Thus, the supplier is unable to maximize either the throughput of the rod mill or the quality characteristics (microstructure) of the rod. This non-optimal operation translates into increased production costs and/or reduced quality levels.
Additionally, the supplier's ability to minimize cost by using cheap raw materials is limited by the buyer's designated chemistry. Typically, a rod may be produced from a variety of chemistries, but with substantially the same mechanical properties. As certain elements are more expensive than others, it is preferable to maximize the use of the cheapest elements (including scrap) while maintaining the integrity of the rod's mechanical properties. However, when the buyer designates the chemistry, the supplier is unable to maximize the use of inexpensive elements within the rod. Thus, the rod may be composed of unnecessary percentages of more expensive elements. A particular chemistry may further prevent the supplier from using scrap raw material if this scrap includes an unduly high percentage of any element.
Heretofore, models have been proposed for simulating various aspects of the rod mill process including the model suggested in "Empirical Models for Predicting The Mechanical Properties of Reinforcing Bar" by O. Delvecchio and C. Young, published October of 1985 in the I & SM. Delvecchio suggests that knowledge of the rod chemistry alone may be insufficient for predicting the mechanical properties of reinforced bar. In Delvecchio's model, yield strength equals the sum of all of the element percentages, each of which is multiplied by a corresponding coefficient. However, each of Delvecchio's yield strength components affords a linear relation to the percentage content of the corresponding element. Delvecchio's model further considers the effect upon the yield strength by the type of steel making facility (e.g., electric arc, basic oxygen, etc.). However, the factor accounting for the facility type merely adds a constant yield strength value to the overall prediction for a particular steel mill (i.e., 16.7 MPa for the "Edmonton" facility which uses an electric arc furnace, and 46.3 MPa for the "McMasters" facility).
Other empirical models have been proposed, such as "Mathematical simulation of Stelmore Process" by R. D. Morales, A. Lopez G., and I. M. Olivares, Ironmaking and Steelmaking, 1991, Vol. 18, No. 2; "Novel Model For Accurate Calculation of Hardenability and Continuous Cooling Transformation", by R. J. Mosterr and G. T. van Rooyan, Material Science and Technology, September 1991, Vol. 7; and "Microstructural Engineering Applied to the Controlled Cooling of Steel Wire Rod: Parts I, II and II", by P. C. Campbell, E. B. Hawbolt and J. K. Brimacombe, Metallurgical Transactions, Vol. 22A, November 1991. Each of the above papers are incorporated by reference. However, none of these models address rod chemistry in combination with a rod manufacturing process.
The need remains within the industry to provide an alternative method and apparatus for producing high carbon rods, in which the supplier is afforded more flexibility with respect to the chemistry of the rods. The present invention is intended to meet this need.